I think they’re just uncomfortable with the word “master”

1 person over at Microsoft complained, and they moved mountains for this person to replace master with main. It sounds like a joke, but it's not.

and that seems completely reasonable to me

No it doesn't. Why does an entire industry need to flip over, because of a single person? Like the ability of changing the master branch for yourself should have been enough. Changing the default over on Github to strong-arm the rest of the world is disgusting behaviour. Which is why I'm sticking to master wherever I can.

especially when they’re people from a group which has been subjected to slavery.

That is literally every group... Every group has been slaves (and slavers) at some point in time. That's not a good argument.